What is the greatest common factor of $21$ and $12$ ? Another way to say this is: $\operatorname{gcf}(21, 12) = {?}$
Answer: The greatest common factor is the largest number that is a factor of both $21$ and $12$ The factors of $21$ are $1$ $3$ $7$ , and $21$ The factors of $12$ are $1$ $2$ $3$ $4$ $6$ , and $12$ Thus, the greatest common factor of $21$ and $12$ is $3$. $\operatorname{gcf}(21, 12) = 3$